A battery pack may supply power to propel a vehicle. Alternatively, the battery pack may receive or sink power when the vehicle is decelerating. When a battery pack is sinking or sourcing current it has continuous current sinking and sourcing capacities as well as transient sinking and sourcing capacities. If a battery pack is supplying a relatively large amount of current during vehicle acceleration, the battery may degrade if the battery current is supplied in an amount greater than a threshold amount of current for longer than a specified time.
One way to ensure that battery does not sink or source an amount of current that is greater than the threshold amount is for the battery pack to output a signal that represents an available current limit to a vehicle controller. By knowing the battery's available current limit the vehicle controller can limit current drawn or delivered to the battery by an electric motor or inverter. In this way, the possibility of battery degradation related to a quantity of current a battery sinks or sources may be reduced.
However, even though a vehicle controller may reduce current demand on a battery in response to an available current limit signal, the vehicle driver may perceive changes in wheel torque that may be objectionable. For example, if a battery pack initially has an available current limit that corresponds to the battery's transient current limit, and the available current limit is later reduced to the battery's continuous current limit as the battery sources current at the transient limit for a prescribed time, the vehicle driver may perceive a wheel torque reduction when the available current limit changes from the transient current limit to the continuous current limit. Thus, vehicle drivability may be affected if a vehicle wheel torque is adjusted in this manner.
The inventors herein have recognized the above issues and developed an approach to address them. In one approach, a method for providing notice of battery pack availability, comprising: sourcing or sinking an amount of current from a battery pack; providing an indication of an available current limit for a battery pack; and providing an indication of a capability of said battery pack to sink or source the amount of current at said available current limit.
By providing an indication of a current limit and an indication of a capability of a battery to sink or source the amount of current at the available current limit, vehicle drivability issues related to dynamic changes in a battery pack's ability to sink or source current may be mitigated. In one example, a buffer signal is provided to a vehicle controller so that the vehicle controller can anticipate when the battery's current sourcing or sinking capability will change. The buffer signal can be related to a history of battery charge/discharge current, the present battery charge/discharge current, the transient charge/discharge current, and a ramp rate for transitioning from a transient current level to a continuous current level. Based on the signal indicating the battery available current limit and the signal representing the battery capability to supply current at the current limit, a vehicle controller can judge when to begin transitioning from a transient current level to a continuous current level. For example, a vehicle controller may determine when to limit current supplied to or drawn from a battery in response to the indicated available current limit and the indicated capability of the battery pack to sink or source current. In particular, if the battery current limit is high, but the battery capability to sink or source current is low, the vehicle controller may gradually (e.g., by ramping motor current) limit current flowing to or from the battery at a first rate. In another example, the vehicle controller may gradually limit battery current at a second rate depending on the rate of change in battery current sourcing or sinking capability as well as the present battery current sinking or sourcing capability. In another example, a vehicle controller may provide an extended amount of time (e.g., by ramping motor current at a third rate) for which current may be sourced or sunk by the battery when the level of battery current is above the continuous current limit but while the battery current is well below the available current limit. In this way, an available current limit signal and a battery capability signal may be used to improve vehicle drivability.
The present description may provide several advantages. In particular, the method can reduce battery pack degradation without having to replace hardware or rely on external systems. Further, the present description provides for additional measures of battery pack degradation protection. Further still, the level of threshold currents can be adjusted for different capability battery packs without having to change hardware and/or software.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.